attachment vs third class

attachment

noun
  • A file sent along with a message, usually an email. 

  • The means by which something is physically attached. 

  • Taking a person's property to satisfy a court-ordered debt. 

  • A device attached to a piece of equipment or a tool. 

  • The act or process of (physically or figuratively) attaching. 

  • A strong bonding with or fondness for someone or something. 

  • The act or process by which any (downward) leader connects to any available (upward) streamer in a lightning flash. 

  • A dependence, especially a strong one. 

third class

noun
  • A reduced-rate mail service for printed matter. 

  • The cheapest accommodation on a train or ship. 

How often have the words attachment and third class occurred in a corpus of books? (source: Google Ngram Viewer )